This invention relates to charge-coupled devices (CCD) and especially to two-phase, charge-coupled, metal-oxide semiconductor (MOS) devices with overlapping polysilicon gates.
Two-phase CCD's in the form of polysilicon overlapped by aluminum (see "Two-Phase Charge-Coupled Devices with Overlapping Polysilicon and Aluminum Gates," Kosonocky and Carnes, RCA Review, Volume 34, March 1973, Page 164) have proved to be viable structures capable of high yield and excellent operation in devices such as analog computers where they can be used as shift registers, for example. High packing density is possible without using stringent manufacturing tolerances. However, these devices have a basic limitation when used in area arrays due to the resistance of the polysilicon gates. Clock pulses are delayed by the RC time of the poly gate relative to the delay down the aluminum clock lines. In order to minimize this effect, the polysilicon is grown as thick as possible which cuts down the light which reaches the silicon substrate thereby reducing sensitivity (the charge in these devices can be transferred by an electrical signal or a light signal). Area imagers made in this fashion will be limited to about 200 vertical TV lines (assuming polysilicon resistance of 50 .OMEGA./.quadrature.).
Also, if aluminum gates are used and it is desired to transfer charge by means of light signals, the array must be illuminated from the bottom (assuming that the top is where the aluminum is located).